1. Field of the Invention
This invention relates to a railroad wheel that exhibits little or no deformation, buckling, or distortion so as to cause the wheel to become out of gauge when a brake shoe is applied thereto, even if the wheel is used to support a heavily loaded car. Furthermore, the invention relates to a wheel adapted to be safely used in a high speed passenger train, and especially a wheel for a high speed tilt-type train that travels at speeds of up to 240 km/h and greater, such as has been proposed for use on Amtrak's Northeast corridor.
2. Discussion of the Background
Conventional railroad wheels include a rim (or tread), a hub, and a plate (a term used in the United States, but described as a "web" in the United Kingdom) that connects the hub and the rim. To lighten the wheels, a process that necessitates minimizing the stresses generated in the wheels, especially stresses resulting from overheating caused by the braking action of a brake shoe rubbing on the wheel, a curvilinear shape with an inflection point near the rim is given to a meridian line of the plate. This is especially the case for the railroad wheel described in French patent application FR 2,687,098.
Stress and creep generated in the wheel by braking actions causes the plate to deform, and become out of gauge, which modifies an axial position of the rim in relation to the hub so that the back-to-back distance between the two wheels on the same axle is either increased or reduced, depending on the geometry of the wheels. When the out of gauge condition decreases the spacing of the rims, track equipment crossing, such as shunting, is made difficult, and can lead to derailments if the wheel is too significantly out of gauge. Therefore, it is preferable to choose a wheel geometry that is aimed at separating the rims when the wheels become out of gauge. However, with this later wheel type, the wheel deformation tends to make the flanges on the wheels grind on the edges of the rails and cause additional wear.
In both wheel types, to avoid the problems generated by the wheels being out of gauge, restricting a weight load per axle during running of the wheels limits overheating of the wheels. Limiting the loads is particularly employed on railroad lines having a layout that requires long braking actions. This is the case, for example, of train lines that go through passes in mountainous regions.
Hot buckling during braking periods is distinguished from residual buckling which occurs after the wheels have cooled and results in the wheels becoming permanently out of gauge as a consequence of overheating the wheels. Residual buckling also depends on a geometry of the wheel, and thus a wheel geometry should be chosen, as determined by the present inventors, that minimizes residual buckling.
A wheel described in patent application FR 2 687 098 has low residual buckling because of its design. But, as determined by the present inventors, this wheel has, nevertheless, like all lightened wheels, a sensitivity to becoming out of gauge that considerably limits the operating conditions under which it can be used. These limitations make this wheel unsuitable for use on heavily loaded freight cars, particularly on mountainous routes.
In addition, consecutive overheating actions generate relatively significant residual tractive stresses in the wheel. Consequently, cumulative tractive stress on these wheels limits their service life and would create a safety hazard if used too long.
The out of gauge problem and, more generally, that of geometric stability in the service life also exists for high speed passenger trains, particularly for wheels intended for tilt-type trains. In fact, these trains that travel at speeds approaching, and even exceeding, 240 km/h, on the one hand, travel on tracks with signaling for two-way working; that is to say, the layout of these tracks is not specifically configured to support high speed travel; on the other hand they are equipped with a mixed braking system employing disks installed on the axles and shoes. Because of these operating conditions, it is preferable, as determined by the present inventors, for the wheels to have a plate, with a curved meridian line, to give them enough radial and axial elasticity so that they will experience the least possible deformation under the effect of successive braking actions or under the effect of very significant braking actions, in particular, during emergency stops. These conditions are necessary to provide train safety. This elasticity and this dimensional stability, however, are not adequate, as determined by the present inventors, for this type of train. In fact, stresses to which the wheels are subject, can generate catastrophic fatigue failures, failures that must be avoided at all cost, since they are very dangerous, and pose a danger to human life when the train is traveling fast.